Optical disc devices are optical information recording/reproducing devices uses of which have been greatly increased by reasons of non-contact type, a large quantity of information per unit volume, high-speed access, low cost, etc., and making the best use of these features, various recoding media have been developed. For example, there have been developed compact disc (CD), laser disc (registered trade mark, LD), CD-ROM, DVD-ROM and the like for reproducing previously recorded information as sounds, images or computer programs, CD-R, DVD-R and the like which are capable of writing information thereon with laser only once and reproducing the information, and magneto-optical disc (MO), DVD-RAM, DVD-RW and the like which are capable of repeatedly recording/reproducing information.
As optical system devices for carrying out recording and/or reproduction of information by the above optical information recording/reproducing media, various devices are known, and as one of them, a rewritable magneto-optical disc device is widely known. For example, a rewritable magneto-optical disc device having a light pickup device in which a ½λ wave plate (also referred to as a “½ wave plate” hereinafter) is arranged midway an optical path where a light emitted from a laser beam source passes through a polarizer and a polarized beam splitter (PBS), reaches a magneto-optical disc and reflected by the magneto-optical disc and the reflected light passes through the PBS again and reaches a photodetector, or a rewritable magneto-optical disc device having a light pickup device in which a ¼λ wave plate (also referred to as a “¼ wave plate” hereinafter) is arranged is known.
The “½ wave plate” used herein is a wave plate that gives an optical path difference of λ/2 (therefore, a phase difference of π) between two polarized light components of specific wavelengths intersecting at right angles, and the “¼ wave plate” used herein is a wave plate that gives an optical path difference of λ/4 (therefore, a phase difference of π/2) between two polarized light components of specific wavelengths intersecting at right angles.
A wave plate used in a liquid crystal projector is, for example, a wave plate functioning as a polarization transfer device and having a function of separating an incident natural light into a P polarized light and a S polarized light whose planes of polarization intersect at right angles and rotating one of the planes of polarization of the thus separated P polarized light and S polarized light substantially by 90° to coincide the angle of the plane of polarization of the polarized light with the plane of the polarization of the other polarized light. According to such a polarization transfer device, most of the polarized lights obtained are made to have a substantially single plane of polarization, so that high utilization efficiency of light can be obtained in the liquid crystal projector. In the polarization transfer device, a ½ wave plate is used as a means to rotate the plane of polarization of the incident polarized light substantially by 90°. In case of the liquid crystal projector, after a light is dispersed into three primary colors of light (RGB) by a dichroic mirror, they are transmitted by the corresponding liquid crystal panels, respectively, then synthesized by a cross prism and released from a projection lens. For the purpose of enhancing luminance, a ¼ wave plate is sometimes placed between the liquid crystal panel and the cross prism, and in this case, a ¼ wave plate making ¼ wavelength not only at the specific wavelengths but also in a wide band is sometimes desired because the wavelength of the dispersed light has a certain width.
As the wave plates applied to such uses, those of inorganic type, such as a wave plate formed from single crystals having birefringence property such as mica, quartz, quartz crystal, calcite, LiNbO3 and LiTaO3, a wave plate having a birefringence film on a surface of a base substrate such as a glass substrate, said film being obtained by depositing an inorganic material obliquely against the base substrate, and a wave plate having a LB (Langmuir-Blodget) film of birefringence property, have been heretofore employed.
Further, there has been employed a wave plate wherein a retardation film, which is an organic substance thin film imparted with birefringence property (function of giving phase difference to a transmitted light) by stretch-orientating a film of a transparent resin, such as polycarbonate (PC), triacetyl acetate (TAC), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyethylene terephthalate (PET), polypropylene (PP), polyallylate, polysulfone, polyether sulfone or acrylic resin, is bonded to a glass substrate or sandwiched between two glass substrates for the purpose of maintaining flatness or fixed shape. Furthermore, there has been also employed a wave plate imparted with birefringence property, in which a high-molecular weight liquid crystal film is formed on a glass substrate or sandwiched between two glass substrates for the purposes maintaining flatness or fixed shape and carrying out molecular orientation.
Recently, as a high-density information recording medium, DVD is rapidly spreading, and on the other hand, optical discs of reproduction only type or rewritable type, such as CD, CD-ROM and CD-R, have already spread widely in the market. For the optical disc devices, therefore, not only an ability of recording information on these various optical discs of different systems but also an ability of reproducing information from them is required. Moreover, with widening of the applicable field, miniaturization and cost lowering are also required. In order to satisfy these requirements, use of wide-band wave plates (retardation plates) applicable to plural lasers for reading and writing has been proposed (patent documents 1 to 3). Of these documents, the patent document 3 (Japanese Patent Laid-Open Publication No. 14228/2002) proposes a wave plate having properties that after two kinds of incident linear polarized lights whose planes of polarization are parallel with each other pass through the wave plate, the planes of polarization of two kinds of outgoing linear polarized lights are made to intersect at right angles. In such a wave plate, two or more retardation films are used in order to obtain desired optical properties, so that not only the retardation film is bonded and fixed to a glass substrate but also the retardation films are bonded and fixed to each other. In case of the wave plate having a constitution that the retardation films are bonded to each other, however, there resides a problem that in-plane aberration is changed by long-term continuous use and excellent properties initially obtained cannot be retained. In addition, there has been pointed out a problem that properties of the retardation films are reflected as properties of the wave plate, so that a phase difference value (retardation) of the wave plate is gradually changed by long-term continuous use depending upon the usage environment, or in-plane aberration is made large by non-uniform thickness of the film, and as a result, excellent properties initially obtained cannot be retained occasionally.
In order to inhibit the above problems, it can be thought that the retardation films are independently incorporated into a laser optical system without bonding them. In this case, however, other problems take place. For example, it becomes essential to precisely control optical axes of the retardation films before they are incorporated, and as a result, much labor is required in the production of equipment. Moreover, deviation occurs in the relation between the optical axes of the retardation films because of vibration or the like given when the wave plate is used, and as a result, properties of the wave plate are lowered.
Patent document 1: Japanese Patent Laid-Open Publication No. 101700/2001
Patent document 2: Japanese Patent Laid-Open Publication No. 208913/2001
Patent document 3: Japanese Patent Laid-Open Publication No. 14228/2002